The present invention relates to a method of disposing of various kinds of slags produced in iron and steel making processes and, more particularly, to a slag disposal method which permits an efficient recovery of concentrates rich in iron for further use in iron or steel making processes.
Heretofore, most slags produced in furnaces used in iron and steel making processes were disposed of. As a result, iron components in the slags which can be recovered were wasted.
In recent years, however, because of the demand for effective use of mineral resources, attempts have been made to recover concentrates in the slags which are rich in iron. More specifically, it has been proposed to recover the iron content in the form of concentrates directly from crude slages produced in furnaces used in iron or steel making processes by magnetic sorting. However, this method has been found to be inefficient since the slags are large in size and of low iron content. Therefore, as a further step to increase the iron content of the concentrates for use in iron or steel making processes, the stored slags have been reduced in size through grinding by means of a rod mill or an autogenous grinding mill. These proposals and attempts are disclosed, for example, in Japanese Patent Publication No. 33047/1976, Japanese Patent Laid-Open No. 147416/1976, Japanese Patent Laid-Open No. 151615/1976, and Japanese Patent Laid-Open No. 33163/1977.
In general, the known prior art slag disposal methods can be roughly summarized as follows.
(1) The maximum size of the crude slags to be disposed of is usually about 300 mm and the particle size, in some cases, has a value ranging between 300 mm and 500 mm.
(2) Slags of sizes smaller than 300 mm and having a high iron content of 50 to 60 percent are usually used directly as concentrates or after increasing the iron content up to 90 percent or higher by a rod mill or an autogenous grinding mill.
(3) Slags smaller than 300 mm and having a low iron content are subjected to magnetic sorting and sieving after crushing so as to provide concentrates directly or after an increase of the iron content through grinding by means of a rod mill or an autogenous grinding mill.
(4) Slags of sizes greater than 500 mm are sorted to determine the iron content by means of a lifting magnet or by visual check and only the portions having a low iron content are subjected to crushing into sizes smaller than 300 mm and then to various disposal treatments.
(5) Slags of sizes greater than 300 mm and having high iron content are often left without any treatment and, when disposed of, require extensive labor and heavy equipment. As recognized, slags of this size and iron content are often bulky and difficult to reduce in size by methods described above. As a result, the conventional disposal method for these slags often involves the first step of reducing size by means of dropping a weight of 2 to 5 tons on the slags. The slags can also be reduced by cutting by a gas burner or breaking by dynamite placed in holes drilled in the slags. Clearly, extreme care must be used due to dangerous scattering of iron fragments.
There is an increasing demand for development of a safe, less expensive method which permits efficient disposal of large masses of slags having high iron content to recover concentrates for further use in iron and steel making processes.
To meet this demand, Applicants have conducted various studies and experiments. In one such experiment, compressive force was applied to slags greater than 500 mm and having a higher iron content and the following results were observed.
(1) With pig iron, slags having iron content approximating 100 percent were roughly crushed and separated from the pig iron itself and from the slags attached to the pig iron or involved by the pig iron. The slags attached to the pig iron or involved by the pig iron had sizes which ranged in the smaller size section of the distribution range of the particles of roughly crushed pig iron.
(2) In the case of steel, slags attached to the steel or involved by the steel were separated as a result of a deformation of the steel by compressive force. Slags also were separated from thin portions of steel and from the defective parts, such as concavities and convexities in the steel surface or shrinkage holes in the steel.
(3) The iron content of the slags roughly separated in paragraphs (1) and (2) above was greater than the iron content of the slags attached to or involved by the metal; in some cases, iron content as high as 90 percent or higher was obtained.
In general, it is conventionally considered that metal in slags cannot be separated from the slags by crushing. Applicants have concluded, however, that the metal in slags does not exist in a homogeneous state, such as in steel sheets, cast steel or cast iron, but has many surface convexities and concavities, as well as cracks. In some cases casting defects such as shrinkage holes and blow holes are involved. Therefore, when compressive stresses are concentrated on the surfaces and internal defects of the metal so that the metal can be crushed by a force which is only a fraction of the compressive strength of the metal in the homogeneous state, it is clear that the force necessary for crushing the slags becomes smaller as the ratio of the cross-sectional area of the slags to the whole crushing cross-sectional area of the mass of slags and metal increases.